A crash has been reported with sierra driver on disconnect with
Ubuntu/Lucid distribution based on kernel-2.6.32.
The cause of the crash was determined as "NULL tty pointer was being
referenced" and the NULL pointer was passed by sierra_indat_callback().
This patch modifies sierra_indat_callback() function to check for NULL
tty structure pointer. This modification prevents a crash from happening
when the device is disconnected.
This patch fixes the bug reported in Launchpad:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/511157